Thermostatic switch



May 8, 1962 T. GELLER THERMOSTATIC SWITCH 2 SheetsSheet 1 Filed May 11,1959 MUM,

ATTORNEY y 8, 1962 T. GELLER 3,033,960

THERMOSTATIC SWITCH Filed May 11, 1959 2 Sheets-Sheet 2 INVENTOR. THOMASL. 6 ELLER mWW ATTORNEY United States Patent 3,033,960 THERMOSTATICSWITCH Thomas L. Geller, West Bend, Wis, assignor to The West BendCompany, a corporation of Wisconsin Filed May 11, 1959, Ser. No. 812,3561 Claim. (61. 200-138) This invention relates to a thermostatic switchwhich is particularly adapted for use in an electric heating circuit forautomatic coffee percolators and similar appliances.

The object of this invention is to provide a thermostatic switch of thistype which is simple and relatively inexpensive to manufacture and,although of small size, is durable and can be easily and accuratelycalibrated.

Another object is to provide a switch having the above advantages whichis highly sensitive to provide accurate control of the heating circuitin which it is used.

These objects are achieved by a switch comprising a tubular body closedat one end with an insulating ringshaped member with an axial openingmounted in the open end of the body. A pair of elongated bimetalliccontact elements are mounted in the member and extend inside the tubularbody. A wedge-shaped spacer is inserted into the axial opening in themember between the bimetallic elements to hold them securely in a pre-,determined position and thereby provide the desired thermal calibration.An adjusting screw can be provided to provide more accurate calibrationif desired.

The thermostatic switch is not designed to respond directly tovariations in liquid temperature as is generally the case. With thecontacts closed, current will flow through the bimetallic elementscausing a temperature rise in the elements due to the resistance of theelements themselves. The heat generated will deflect the elements andopen the contacts at a predetermined ambient operating temperature. Thebimetallic elements are small and therefore cool quickly permitting thecontacts to close again providing a cycling action. The percentage ontime of the switch will -vary with the rate of flow of heat from thebimetallic elements into the surrounding mass which, in turn (uponproper positioning of the switch), will vary with the temperature of theliquid in the percolator. Thus, the lower the temperature of the liquidthe greater rate of heat flow from the elements and the greater the ontime of the switch.

Other objects and advantages will be pointed out in, or be apparent fromthe specification and claim, as will obvious modifications of the singleembodiment shown in the drawings, in which:

FIG. 1 is a plan view of a plug-in type unit for an electric percolatorembodying the thermostatic switch of my invention;

FIG. 2 is a section taken along line 2-2 of FIG. 1;

FIG. 3 is a fragmentary sectional view taken through a percolatorconstructed to receive the thermostatic unit shown in FIGS. 1 and 2;

FIG. 4 is an enlarged plan view of the thermostatic switch of thisinvention;

FIG. 5 is a sectional view taken along line 55 of FIG. 4;

FIG. 6 is an end view of the switch shown in FIG. 4; and

FIG. 7 is a partially schematic and simplified wiring diagram of athermostatically controlled heating circuit using the switch of myinvention.

Referring to the drawings (FIGS. 4,5 and 6), the thermostatic switchincludes a tubular body 19 preferably of metal having one end closed asshown. A pair of flat elongated bimetallic elements 12, 12 with contacts14, 14 are mounted in body it) by a ring-shaped member 16 3,d33,%0Fatented May 8, 1962 ice having a flanged end 17 and a serrated outersurface 19. The ring is inserted into the open end of the body with theannular shoulder formed by flange 17 abutting the end of body it} toposition it in the body. Bi-metal elements 12, 12 extend through theaxial opening in the ring and are held securely in assembled positionagainst fiat walls 18, 18 on the inner surface thereof by a wedgeshapedspacer pin 20 wedged into the ring between the elements as shown. Ring16 and pin 20' are preferably made from molded phenolic plastic, andelements 12, 12 and pin 20 are sealed in ring 16 by epoxy resin 24 asshown.

One or both flat inner walls 18, 18 are tapered slightly (only one walltapered in FIG. 5) and pin 20 is similarly tapered to conform with theangle of the walls to thereby insure a tight fit between ring, elementsand pin. In adition, it is important to note that by the use of a ringand pin assembly of predetermined contour, elements 12, 12 can beuniformly positioned to provide switches having a specific thermalcalibration. That is, by keeping the slope of walls 18, 18 in ring 16constant the pressure at contacts 14, 14 of the switch elements can beheld within close limits.

To permit even more accurate calibration of the switch, a trim screw 22(FIG. 2) threadably mounted in thermostat housing 26 and extendingthrough an opening 23 in body it} is provided to pre-stress the bimetalelements after assembly of the switch.

As shown in FIGS. 1 and 2, the thermostatic switch is mounted in aplug-in type housing 26 and connected by suitable leads 28 to femaleterminals 39, an indicator light 31 and a line cord 32. Thisthermostatic switch assembly is adapted for plug-in connection with apercolator 34 (FIG. 3) having a sealed heating element 36 with maleterminals 38 positioned below a sealed open end recess td in the base ofthe percolator. The electric heating circuit (FIG. 7) controlled by thethermostatic switch is completed for operation of the percolator byplugging the switch assembly into the appliance with terminals 38 matingwith terminals 34 and body 10 of the switch extending into cavity 4i Asshown in FIG. 7, with contacts 14 on the bimetal elements closed (asthey normally are), curent will flow through the switch to heatingelement 36. The ohmic resistance of bimetal elements 12 is such that ata predetermined ambient operating temperature the current passingthrough them will produce sufficient heat to cause them to deflect andopen contacts 14 and thereby deenergize heating element 36. Elements 12being small (.025 inch thick X .156 inch wide x .750 inch effectivelength in preferred embodiment) will cool quickly causing the switchcontacts to close and again energize heating element as and againinitiate the self-heating action of the elements. Thus, the switch willcycle on and off at a given ambient operating temperature in response tothe heat produced by the current fl w through the ole ments and thesubsequent cooling of such elements when the contacts open. The amountof heat produced by the heating element during a given period of timewill, of course, depend on the percentage of such period of time theswitch is closed. The on time of the switch during which the heatingelement is energized will depend on the length of time it takes theelements to cool, i.e. the rate of heat flow from the elements into thesurrounding mass. By positioning the switch in cavity 40 in closeproximity with the liquid in the percolator, the rate at which elements12 will cool can be made to vary with the temperature of the liquid. Thelower the temperature of the surrounding mass the greater the heat flowfrom the elements and the greater the percentage of on time in theoperation of the switch. It has been found that by properly balancingthe length, width, thickness, thermal activity and electricalresistivity of the bimetal elements in conjunction with the physicallocation of the switch with respect to a given mass and wattage to becontrolled, a highly sensitive and accurate control can be designed.

Although but one embodiment of the present invention has beenillustrated and described, it will be apparent to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the spirit of the invention or from the scope of theappended claim.

I claim:

A thermostatic switch comprising, a hollow body closed at one end, aninsulating member mounted in the open end of said body and having anopening therethrough, a pair of elongated resilient contact elementsmounted in said opening and extending inside said body, at least one ofsaid elements being a bimetal element, said elements being straight overthe length thereof and each having a contact mounted thereon, a spacerinserted into the opening in said insulating member between saidelements to hold said elements securely in a predetermined position withrespect to each other, said opening in said insulating member having apair of oppositely facing flat walls against Which said contact elementsare held by said spacer, at least one of said flat Walls sloping fromone end of said member to the other, said spacer being tapered toconform to the angle of said fiat Walls of said member whereby saidelements will assume a non-parallel pre-stressed relationship havingsaid contacts thereon in contact with each other when the switch issubjected to normal ambient temperature conditions.

References ilited in the file of this patent UNITED STATES PATENTS

